This invention relates to improvements in fluid condition monitoring apparatus and methods of monitoring the condition of a fluid. It is especially, but not exclusively, suited to the measurement of the condition of engine oil such as its shear impedance (or viscosity/density) and temperature and the detection of changes in oil condition as the oil ages over time.
The condition of the oil in an engine, for example an engine of a motor vehicle, is important for the reliable operation of the engine. With recent trends in vehicle servicing causing the number of operating hours of the engine between services to increase, the oil in an engine remains in the engine for longer periods of time than ever before. Over this time the oil may deteriorate due to, for example, sooting or other particulate contamination, fuel dilution, oxidation or shear thinning. Clearly the rate at which the oil deteriorates will depend on the age of the engine and the way in which the engine is operated between services. Attempts have been made by many vehicle manufacturers to monitor the operation of the engine allowing variable service intervals to be employed.
A key property of an engine oil is its viscosity—essentially its ability to resist shear forces. This is an essential property of an oil when protecting the component parts of an engine from premature wear. When an oil acts as a lubricant between two bearing surfaces operating at high frequency the shear rates are high and so it is also logical to measure the shear viscosity of the oil at high rates of shear. This can be measured in a variety of ways. For example, U.S. Pat. No. 4,721,874 teaches the use of a transducer which oscillates at high frequency to send ultrasonic shear waves into the fluid through a wave guide. A portion of the wave will be reflected back onto the transducer at the interface between the guide and the fluid. When this reflected wave is incident upon the transducer it produces an output signal. A measurement of the amplitude of this output signal gives an indication of the shear rate (or more correctly the product of viscosity and density). Whilst this offers a useful measurement of viscosity it does not provide any information, which would enable the cause of the change to be determined. Effects such as sooting, oil dilution, oxidation and shear thinning all alter the viscosity of an oil in different ways. The applicant has appreciated that it would be advantageous to be able to distinguish between the different effects as well as measuring simple changes in viscosity. This would provide valuable information about the overall condition of an engine and not just its oil, perhaps allowing a warning to be raised of any wear or engine damage.